When powerful processor machines with a very low failure rate were first introduced into telephone exchanges, more than ten years ago, their introduction went hand-in-hand with a centralization of substantially all control functions. Such processors could process telephone events in real time, and further they could be programmed to provide a rich variety of new services and facilities.
However, distributed control systems are preferable from the points of view of exchange flexibility and modularity, and new factors which have occurred over the last few years are now facilitating the design of time division exchanges which rely on distributed control. The main factors are as follows:
The development of powerful microprocessors together with their associated LSI circuitry and software tools for developing programs therefor;
The possibility of using the switching network to switch paths for control signals in addition to speech paths over digitally multiplexed links; and
The exchange can switch both speech paths and computer services by means of appropriate microprocessor controlled terminals.
In known control systems using a plurality of processors, the processors are often connected to one or more common bus lines, which are also connected to various common resources such as peripheral memories.
These arrangements are difficult to manage and they are vulnerable. Further, the number of processors that can be accommodated in a single set is fairly limited. Such a structure is not appropriate for a telephone exchange driven by a pool of microprocessors and equipped with microprocessor controlled terminal units, since such an exchange could easily require thirty or so processors connected to a hundred or so terminal units.
Preferred embodiments of the present invention provide a telephone exchange driven by a plurality of processors which do not share a common bus, which do not have direct access to common resources, but which communicate with each other by a system of sending messages.
Such exchanges can be highly modular in design and the equipment can be provided in a carefully designed redundant manner, whereby faults affecting only a small part of the exchange can be dealt with automatically, so that faults requiring emergency human intervention are extremely rare.